RAPID RECOVERY OF STRUCTURE AND FUNCTION OF THE CHOLINERGIC SYNAPSES IN THE CAT SUPERIOR CERVICAL-GANGLION IN-VIVO FOLLOWING STIMULATION-INDUCED EXHAUSTION

Cat superior cervical ganglia (SCG) were tetanically stimulated in viv o at 30-100 Hz until neural transmission was exhausted, and then were allowed to rest and recover. Changes in their cholinergic synapses wer e examined electrophysiologically and morphologically during the time of tetanic stimulation and during recovery. For morphometric analysis the presynaptic terminal was subdivided into two areas: an area direct ly over the active zone, termed zone-I, (bounded by a hemicircle with a diameter equivalent to the active zone length), and the remaining pr eterminal area, termed zone-II. In control ganglia before stimulation synaptic vesicle density in zone-I (SVD-I) averaged 90 mum-2 and the n umber of vesicles actually attached to the active zone (SVA) averaged about 2.5 per single profile of nerve terminal. Upon stimulation, the postganglionic potential immediately began to decline in amplitude and disappeared after 1 min of stimulation. Simultaneously, SVD-I decline d to less than 35 mum-2 and SVA declined to less than 1 per section. T hereafter, stimulation was terminated and the ganglion was allowed to rest. Recovery of the postganglionic potential was monitored by stimul ation at 1 Hz. The postganglionic potential reached control levels aft er only 1 min of rest. Likewise, the structural parameters, SVD-I and SVA, also rapidly recovered, reaching control levels after only 30 sec of rest, slightly faster than the postganglionic potential. This illu strates that stimulation-induced fatigue of transmitter output and dep letion of synaptic vesicles recover to the control level at a high rat e in synapses of the cat SCG with a normal supply of blood. In fact, m orphological recovery may be slightly faster than electrophysiological recovery. Mechanisms of vesicle formation and migration to the presyn aptic area are discussed in light of these observations.